Present electronics technology has allowed an increased number of sophisticated devices to be made portable. Accordingly, there is an increased demand for power systems for such portable electronics. Typically such power sources come in the form of electrochemical battery systems which may be disposable or rechargeable. From an economic standpoint, rechargeable systems offer an advantage over non-rechargeable systems since such batteries may be used and recharged hundreds of times. This fact has been reflected in the marketplace since virtually all advanced electronic devices come with rechargeable battery systems when purchased.
Some of the more common rechargeable systems include nickel-cadmium (NICD), nickel metal-hydride (NIMH), and sealed lead acid (SLA). Recently, however, other systems have appeared that are promising as well; these include lithium ion (LiION) and lithium solid state/polymer (LiSS). All of these systems are ideally recharged by applying a recharge current that, at least for a portion of the recharge cycle, is held at a constant level. The ideal level is determined by the chemistry involved, as well as the capacity of the battery cells. To illustrate this point, compare NICD and LiSS. NICD technology is very mature, and consequently, robust. NICD cells can handle a significant variation in recharge current levels with minimal effect on the useful life of the cells. As a result, chargers for NICD systems may offer only one recharge current for a variety of capacities. Small NICD batteries are then recharged very rapidly and large capacity NICD batteries take proportionally longer to recharge. LiSS in contrast, is very sensitive to recharge current level. Excess recharge current has a definite negative effect on cell cycle life. However, the increased capacity vs. weight of LiSS makes it a very desirable system for very small portable devices such as cellular phones.
The disadvantage of rechargeable battery systems is that they do require a substantial investment by the consumer compared to the disposable non-rechargeable systems. Accordingly, it is desirable from a marketing standpoint to offer new rechargeable systems in a retro-fit fashion. That is, the newer power source can take advantage of the chargers of the older systems. In this way consumers can enjoy the benefit of the latest battery chemistry systems without necessarily having to purchase a new charger. In order to do this, the recharge current must be controlled such that the cells of a newer battery system receive an optimum recharge current. Any current in excess of this optimum level provided by the charger must be diverted away from the cells in order to prolong the useful cycle life of the battery.
Another consideration is the cost of the charger. Since the recharge current is typically regulated to a constant level, the power regulator circuitry is the most expensive part of the charger. If the current were less regulated the charger cost could be reduced considerably. One example of a less regulated system would allow full wave rectified current pulses to be applied to the battery, resulting in very high peak currents. The result would be that a high current in excess of the optimum recharge current level would be applied every cycle, unless a means were devised to divert the excess current away from the battery cells.
Therefore there exists a need for a recharge current control means which diverts current in excess of a predetermined optimum recharge current level away from the battery cells. This recharge current control could also be retro-fit into an existing battery system charger, in an inexpensive manner.